Graphene is a sheet of a sp-bonded carbon atom with a thickness of an atom, is a six-membered ring sheet such that a benzene ring is laid on a two-dimensional plane, has a hexagonal lattice structure in a plan view such as a honeycomb composed of a carbon atom and a bond thereof, and has an enormous specific surface area. Also, graphene has excellent properties such as high strength, high electrical conductivity, high transparency, and high thermal conductivity.
Meanwhile, graphite is constituted by stacking graphene sheets in multitude so as to contact with a surface of each other, and these graphenes are strongly bonded by Van der Waals force. Graphite is a structure such that plural graphenes are laminated; however, most surfaces of the laminated graphenes contact with each other, and the property of the highly conductive surfaces is not utilized and excellent properties of the graphenes are lost.
Ordinarily, graphene is produced in such a manner that graphene oxide is produced from graphite, chemically peeled off and then reduced to graphene. This is a producing technique for allowing mass production at a low cost. However, graphene is so unstable by reason of a thickness of a carbon atom that produced graphenes are easily rebonded by Van der Waals force in contacting with each other to reproduce graphite. This is a problem in the producing process of graphene.
FIGS. 1(a) and 1(b) are process drawings explaining that produced graphenes are rebonded to reproduce graphite in the process of producing graphene from graphite. FIG. 1(a) is a structural drawing and FIG. 1(b) is a ball stick model.
First, graphite is oxidized in a liquid of mixed acid to produce graphene oxide. The graphene oxide is peeled off into one sheet by expansion in accordance with the oxidation.
The graphene oxide has a carbonyl group, a hydroxyl group and a carboxyl group on the surface. The affinity of these chemically functionalized molecules with a solvent such as water is so high that the peeled graphene oxide is uniformly dispersed into the water and other solvents.
Next, the peeled graphene oxide is reduced by using a reducing agent. Thus, graphene is produced.
However, the affinity of graphene with a solvent such as water is so low, namely, hydrophobic by reason of not comprising these chemically functionalized molecules that graphene may not be uniformly dispersed into the solvent such as water but be aggregated. Graphene is so unstable by reason of a thickness of a carbon atom that the produced graphenes are easily rebonded by Van der Waals force in aggregating to reproduce graphite.
In Patent Literatures 1 and 2, bumps and nodules are conjugated as a spacer on the surface of graphene so that graphene obtained by reduction is not rebonded. However, this constitution does not allow the maintenance of high electrical conductivity on the surface of graphene to be sufficiently secured.
Also, in Patent Literature 3, a graphene sheet film with a carbon nanotube linked is disclosed and it is described that the graphene sheet film may be applied to a graphene sheet capacitor. However, in Patent Literature 3, since the graphene and the carbon nanotube are mixed up directly, only part of the graphene is subject to self-assemble. Also, the constitution is such as to be conjugated by π-π bonding through the carbon nanotube, and is not a direct bond which becomes electrically and mechanically integral, so that the high electrical conductivity on the surface is not sufficiently maintained.
Also, in Non Patent Literatures 1 to 3, the reduction of graphene oxide is disclosed.
In Non Patent Literature 1, graphene oxide is reduced by hot water of 180° C. in an autoclave. In Non Patent Literature 2, hydrazine is used and graphene oxide is reduced by heating to 150° C. In Non Patent Literature 3, graphene oxide is reduced by two stages of hydrazine and ethylene glycol. All of carboxyl groups and hydroxyl groups of an outer edge of graphene oxide are removed on these strong reduction conditions to produce graphene. As described above, graphene is so unstable by reason of a thickness of a carbon atom as to reproduce graphite.
Thus, the fact is that a graphene laminated structure, in which high electrical conductivity on the surface of graphene is maintained, is not obtained, and a new structure, in which the property of high electrical conductivity on the surface of graphene is utilized, and the establishment of a producing method therefor are demanded.
Meanwhile, a high-performance storage device is also demanded for efficiently utilizing of energy.
Among various storage devices, a capacitor, particularly, an electric double layer capacitor is so large in power density as to allow high-speed charge and discharge. However, it has been conceived so far that energy density is small, and the capacitor may not meet large-capacity needs for electric cars and so on.
Graphene with a thickness of a carbon atom is presently larger by far in specific surface area and electrical conductivity than activated carbon powder used for a capacitor electrode to allow capacitor performance to be dramatically improved. Graphene is the largest in specific surface area and electrical conductivity among substances, and is extremely excellent as a capacitor electrode material.
As described above, as shown in FIGS. 1(a) and 1(b), graphene is ordinarily made from graphite. The method is chemical treatment based on Hummers method for allowing mass production at a low cost. In this method, in order to produce a capacitor utilizing the properties of graphene, when graphene oxide is reduced to graphene, the problem is that graphene is hydrophobic and is not to be uniformly dispersed into an aqueous solution, and graphene peeled off into one sheet aggregates again as shown in FIG. 2 and is rebonded to each other to return to original graphite.
A method for making a spacer composed of nanoparticles insert between graphenes is proposed for preventing graphene from being rebonded to reproduce original graphite (Patent Literatures 1, 2 and 4). However, the concept that nanoparticles are used as a spacer is only proposed, and it is not specifically disclosed how to complex nanoparticles or how to utilize nanoparticles as a spacer.
A graphene capacitor electrode used a carbon nanotube as nanoparticles and complexed with graphene is proposed (Patent Literature 3 and Non Patent Literatures 1 to 4).
Here, the carbon nanotube has a role as a spacer for preventing graphene from being rebonded to reproduce original graphite. In addition, the carbon nanotube has a role as a linking agent for linking graphene sheets by π-π bonding. The carbon nanotube and graphene are complexed to form a graphene laminated structure. Incidentally, a term such as a stacked structure is used in Non Patent Literatures 1 and 3, but no description is offered on a specific stacked structure.
FIG. 3 is a perspective view showing an example of a graphene laminated structure using a carbon nanotube (CNT) as a spacer.
Since the carbon nanotube has high electrical conductivity, the graphene laminated structure comprising the carbon nanotube between graphenes has also high electrical conductivity. Thus, a graphene stacked structure utilizing a large specific surface area and electrical conductivity of graphene is made. However, graphene is directly bonded so easily by reason of intermolecular force between graphene sheets as to narrow an interlayer gap and decrease the adsorption amount of an electrolyte. Accordingly, sufficient performance may not be exhibited for being put to practical use as a capacitor electrode. The addition of the carbon nanotube improves the electrical conductivity and serves as a role of an adhesive.
No reports have been made so far about an electrode film having so thick a film thickness as to allow sufficient performance to be exhibited for being put to practical use as a capacitor electrode and having a graphene stacked structure utilizing electrical conductivity of graphene, which has large power density and energy density to allow high-speed charge and discharge.